Wouldn't it be great to have your own cellular tower at home? You'd always have a strong signal on your mobile phone, and you wouldn't be paying to use the carrier's network.

It sounds like a pie-in-the-sky idea, but it's actually the gist of what T-Mobile USA is rolling out Wednesday: the option to use your Wi-Fi router instead of the cellular network on two new T-Mobile phones.

I tried out the system for a few weeks, and found it worked well.

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For those who have poor reception in their homes and would like to drop their landline, T-Mobile HotSpot AtHome appears be a good choice. For others, I'm not sure the extra cost is worth it, particularly since T-Mobile's long-term pricing isn't clear.

This isn't like having a cell phone that also happens to work as a cordless phone. You have the same number, whether you're on Wi-Fi or cellular.

And in an engineering feat, the new phones will hand over calls that are already in progress from Wi-Fi to the cellular network if you leave the hotspot, so you can start a call at home and then keep talking as you walk out.

Even Apple Inc.'s (AAPL) much vaunted iPhone, launching on Friday, doesn't do that, even though it has built-in Wi-Fi, providing a great opportunity to take the presumably proud new owners down a notch. ("So how much did your iPhone cost again? And it can't do seamless handover between Wi-Fi and cellular?")

T-Mobile's phones also automatically connect to the company's 8,500 commercial hotspots in the U.S., including many Starbucks (SBUX) locations.

The technology behind the service is known as UMA, or Unlicensed Mobile Access, and has broad support among cell-phone manufacturers, so we can expect to see more of it. European carriers are already using it.

T-Mobile is the first major U.S. carrier to get on board, after trying it out in the Seattle area since October. Cincinnati Bell launched a similar service last month.

I tested the cellular-to-Wi-Fi handover a dozen times, and now and then noticed a momentary audio drop-off. One call was apparently dropped at handover, but it seems acceptable to have that happen occasionally.

There was no noticeable difference in sound quality between the two wireless technologies, an impressive result considering the often spotty audio yielded by other services that use broadband connections for phone calls. UMA is clearly quite different from standard Voice over Internet Protocol, or VoIP.

The new phones, the Nokia 6086 and Samsung t409, cost $50 with a two-year contract and a calling plan that costs at least $40 month (but remember that taxes and other fees bring the actual cost closer to $50).

They're unremarkable camera phones. I tested the Nokia, which was solid, but has rather poor audio quality overall.

Unlimited free calling on Wi-Fi then costs an extra $10 a month for a single line, or $20 for a family plan with up to five lines.

Those are, however, promotional offers. The regular price for each plan is $10 higher.

The company hasn't said how long the promotional offers will extend, but if you sign up at the lower price, you get to keep it.

The phones will connect to any Wi-Fi router, but for your home, T-Mobile recommends either of two routers it's providing for free, after a $50 rebate.

The one I tested was a modification of the Linksys WRT54G. That's a popular model, but setup was a hassle, as it usually is for routers, with a misleading manual and installation software that didn't work.

The company says the payoff for using its router is threefold:

— It gives calls higher priority, so if you have a computer download going at the same time, your call won't be affected.

I'm not sure how great this benefit is. I used the Nokia phone on my own router while using the computer and didn't notice a problem. The phone's bandwidth demand is quite small.

— It has a button that will allow the phone to connect to an encrypted router without typing in the Wi-Fi password. This is a great feature.

Strangely, I couldn't find any documentation, and had to call the company to learn how to use it, but T-Mobile will no doubt straighten this out and update its manual.

— It quadruples the phone's battery life. I wasn't able to stringently test this claim, but it's clear that even without the T-Mobile router, the Nokia phone did quite well.

Wi-Fi is much more power-intensive than cellular, and I've tested phones before that used only Wi-Fi and generally went dead after 24 hours on standby.

The Nokia phone ran for about two days in mixed cellular and Wi-Fi use with my own router, and three days with the T-Mobile router.

T-Mobile says the phone has up to a week of standby time on cellular, and up to three days on Wi-Fi.

At the promotional price of $10 a month for a single line, I think this is a reasonable value if your home coverage is spotty. T-Mobile, a rather distant fourth in wireless subscriber numbers in the U.S., doesn't have the most extensive network.

The free unlimited calls on Wi-Fi are a nice bonus, but most of T-Mobile's plans already provide free calls on nights and weekends or free calls to your five favorite numbers, so the actual savings are likely to be small.

However, you can increase your savings by using a tip I gleaned from the company: If you start your call on Wi-Fi and then head out onto the cellular network, the whole call is free.

The regular price of $20 a month seems high, especially since you'd be paying T-Mobile to use your own broadband connection for calling, taking the load off T-Mobile's cellular network.

If you already have enough minutes on your cellular plan, what you can do is this: Buy one of the UMA phones, but don't sign up for the monthly Wi-Fi add-on. You'll be able to place calls over Wi-Fi, but they will be counted toward your monthly plan just like cellular calls.

Use your own router — you won't get the rebate that makes the T-Mobile router free if you don't get the Wi-Fi add-on.

UMA could be a real money saver if T-Mobile would combine free Wi-Fi calling with prepaid cellular — the company has cheap rates for low-volume callers.

T-Mobile has gotten a tricky technology to work here. The fact that it doesn't work optimally with regular routers is perhaps its greatest weakness, but by no means a large one.